Photodiode and photosensitive device

ABSTRACT

Provided is a semiconductor photodiode which has an electrode structure having not only high adhesion to a Mg2Si material but also improved overall performance including photosensitivity. A photodiode comprising: a pn junction of a magnesium silicide crystal; an electrode comprising a material that is in contact with p-type magnesium silicide; and an electrode comprising a material that is in contact with n-type magnesium silicide, wherein the material that is in contact with p-type magnesium silicide is a material which has a work function of 4.81 eV or more and reacts with silicon to form a silicide or form an alloy with magnesium.

TECHNICAL FIELD

The present invention relates to a photodiode using magnesium silicide.

BACKGROUND ART

With recent dramatic technological innovation in relation to artificialintelligence (AI) and the like, research and development of a system forautomatic monitoring and controlling in place of human eyes and handsare being energetically carried out. In such automatic monitoring andautomatic controlling systems, an appropriate response operation isdetermined based on various input information such as lights,temperatures and voices, so a hardware for detecting input signals willbe a key device that plays an important role in the entire system.Particularly, in terms of detecting the light input signals, advancedautomatic monitoring can be realized by using a device that replaces thehuman eyes or in some cases can detect information in areas which cannotbe detected by the human eyes.

Devices that are sensitive to optical input signals include those havingelements that convert the optical signals into electrical signalscapable of being processed electronically. Examples of the basicelements include light detection elements using semiconductor materials,and among them, photodiodes using a pn junction is known in the art.

The photodiodes using semiconductor materials have different sensitivewavelength regions depending on band gaps possessed by the semiconductormaterials. In order to perform advanced control that can respond toautomatic monitoring at night, automatic driving of a motor vehicle andthe like, input information regarding a light in an infrared region isrequired in addition to a light in a visible light region andinformation of an image. Therefore, there is a strong need for elementsand devices including photodiodes that can detect light inputs with highsensitivity in the infrared region, so that active studies anddevelopments are being advanced using various semiconductor materials.

Photodiodes using compound semiconductor materials such as InGaAs,HgCdTe and InAsSb are already known as photodiodes that assume detectionof the light in the infrared region having a short wavelength (awavelength of about 0.9 to 2.5 μm). However, these semiconductormaterials have disadvantages of containing rare elements in the compoundmaterials, or being harmful to the living body and having highenvironmental loading. Therefore, the present inventors have prepared acrystalline material of magnesium silicide (Mg₂Si), a compoundsemiconductor composed of magnesium (Mg) and silicon (Si), which is amaterial which is rich in nature as a resource and which is less harmfulto the living body and highly safety (Non-Patent document 1), andproposed and produced a photodiode using the material to provide certainresults (Non-Patent Documents 2 and 3).

As an electrode used for a semiconductor device, a gold (Au) electrodeis often used as described in Non-Patent Document 2, from the viewpointsof conductivity and chemical stability, and further easiness offormation. However, Au is chemically stable, so that it does not easilyreact particularly with Si or Si-based compounds, and tends to have pooradhesion at a contact interface with the semiconductor. Therefore, ittends to generate peeling and separating of the electrode from thecontact interface with the semiconductor, causing a problem ofdurability and operability of the device, in view of a practical useenvironment of a practical device.

To solve such problems, an attempt to improve adhesion at the contactsurface between the electrode material and the semiconductor materialhas been made in the Si-based semiconductor device by interposing othermetal material between the Au electrode and the contact surface with thesemiconductor. Non-Patent Document 3 discloses that titanium (Ti) isinterposed between an Au electrode and a contact surface of Mg₂Si thatis the semiconductor material. It has allowed provision of an electrodehaving an adhesion that would not cause any problem in view of apractical device, even in the photodiode using Mg₂Si as a semiconductormaterial.

However, further subsequent consideration of physical propertiesspecific for Mg₂Si has revealed that Ti is not necessarily best as anelectrode material that is in direct contact with Mg₂Si in order toimprove further the overall device performance includingphotosensitivity in addition to the point of the adhesion between theelectrode and the semiconductor material, and that it is necessary tostudy the electrode material from other viewpoints.

CITATION LIST Non-Patent Literatures

-   Non-Patent Document 1: K. Sekino et al., Phys. Proc., Vol. 11, 2011,    pp. 170-173-   Non-Patent Document 2: T. Akiyama et al., Proc. Asia-Pacific Conf.    Semicond. Silicides Relat. Mater. 2016, JJAP Conf. Proc. Vol. 5,    2017, pp. 011102-1-011102-5-   Non-Patent Document 3: K. Daitoku et al., Proc. Int. Conf. Summer    School Adv. Silicide Technol. 2014, JJAP Conf. Proc. Vol. 3, 2015,    pp. 011103-1-011103-4

SUMMARY OF INVENTION Technical Problem

The technique of the present disclosure is intended to solve the abovetechnical problems. An object of the present disclosure is to provide asemiconductor photodiode using Mg₂Si as a semiconductor material, whichhas an electrode structure having not only high adhesion to the Mg₂Simaterial but also improved overall performance includingphotosensitivity.

Solution to Problem

As basic findings based on the technique of the present disclosure, thepresent inventors have focused on inherent physical property valuesregarding an energy level of Mg₂Si in the process of clarifying thebasic physical properties of the compound semiconductor of Mg₂Si.Detailed physical properties of the compound of Mg₂Si have been littleknown, but as a result of further studies based on the contentsdisclosed in Non-Patent Document 1, the present inventors haverecognized the fact that an electron affinity (an energy leveldifference between a vacuum level and a conduction band) in Mg₂Si is4.51 eV. Then, in view of the inherent physical property values ofMg₂Si, the present inventors have conceived a technical means that canfurther improve characteristics of the Mg₂Si photodiode by appropriatelyselecting the electrode material.

More particularly, as described below, the present disclosure relates toa technique that prevents an energy barrier in a transport direction ofphoto carriers from being generated at a semiconductor/electrodeinterface by appropriately adjusting a relationship between the energylevels of Mg₂Si and the electrode material. FIG. 1 shows an example ofthe basic structure of the Mg₂Si photodiode, and FIG. 2 shows an energylevel diagram for a pn junction and an electrode near-field region in acase where an electrode material 103 directly contacted with a p-typeMg₂Si part 102 is Ti (which is a material also used in Non-PatentDocument 3) and an electrode material 105 directly contacted with an-type Mg₂Si portion 101 is Al in the structure of FIG. 1. Amongelectron-hole pairs which are photo carriers generated in a depletionlayer (and a diffusion length) 201 of the pn junction by lightincidence, an electron 202 moves to a position of a lower energy leveland the hole 203 moves to a position of a higher energy level.

The problem here is a difference in work functions between the p-typeMg₂Si and the electrode. The work function of the semiconductor is avalue obtained by adding to an electron affinity an energy differenceΔE_(F) from a conduction band to a Fermi level. The Fermi level varieswith the carrier concentration in the semiconductor, and has a value ofΔE_(F) between 0 and 0.3 eV in n-type, and a value of ΔE_(F) between 0.3and 0.6 eV in p-type for Mg₂Si. From the electron affinity for Mg₂Si asshown above, the work function of p-type Mg₂Si is between 4.81 and 5.11eV. The work function of Ti which is the material of the electrodeconventionally used herein is about 4.33 eV, which is lower than thelowest work function value of 4.81 eV for Mg₂Si as shown above.Therefore, an energy barrier 204 for electron holes is generated at theinterface between the p-type Mg₂Si and the electrode, which cases aproblem that the proportion of electron holes which can reach theelectrode is decreased among the generated electron holes, and thegenerated photo carriers may not be effectively detected asphotocurrent.

In contrast, as shown in FIG. 3, when the electrode material that is incontact with p-type Mg₂Si is made of a material having a work functionof 4.81 eV or more, the energy barrier for electron holes is eliminatedat the interface between p-type Mg₂Si and the electrode, or an impact ofthe energy barrier can be reduced to a level that would cause nopractical problem, and many of the electron holes of the electron-holepairs generated as photo carriers can reach the electrode, therebyeffectively contributing to a value of photocurrent.

Based on the findings and ideas as above, the present disclosureprovides the following inventions:

1)

A photodiode comprising: a pn junction of a magnesium silicide crystal;an electrode comprising a material that is in contact with p-typemagnesium silicide; and an electrode comprising a material that is incontact with n-type magnesium silicide,

wherein the material that is in contact with p-type magnesium silicideis a material which has a work function of 4.81 eV or more and reactswith silicon to form a silicide or form an alloy with magnesium.

2)

The photodiode according to 1), wherein the material that is in contactwith p-type magnesium silicide is at least one metal selected from thegroup consisting of nickel, cobalt, platinum, palladium, iridium,rhenium, rhodium, beryllium, selenium, and tellurium, or at least onealloy thereof.

3)

The photodiode according to 1) or 2), wherein the material that is incontact with n-type magnesium silicide is a material which has a workfunction of less than 4.81 eV and reacts with silicon to form a silicideor form an alloy with magnesium.

4)

The photodiode according to any one of 1) to 3), wherein the materialthat is in contact with n-type magnesium silicide is at least one metalselected from the group consisting of aluminum, gallium, indium,arsenic, antimony, bismuth, silver, copper, zinc, cadmium, titanium,vanadium, chromium, manganese, iron, yttrium, zirconium, niobium,molybdenum, hafnium, tantalum and tungsten, or at least one alloythereof.

5)

The photodiode according to any one of 1) to 4), wherein the electrodecomprising the material that is in contact with p-type magnesiumsilicide comprises: the material that is in contact with p-typemagnesium silicide; and other material that is in contact with theformer material.

6)

The photodiode according to 5), wherein the other material is at leastone metal selected from the group consisting of gold, palladium andplatinum or at least one alloy thereof, except for the metal selected asthe material that is in contact with p-type magnesium silicide.

7)

The photodiode according to any one of 1) to 6), wherein the materialthat is in contact with p-type magnesium silicide is in a form of a thinfilm having a thickness of from 1 to 1000 nm.

8)

The photodiode according to any one of 1) to 7), wherein the p-typemagnesium silicide is magnesium silicide doped with silver.

9)

The photodiode according to any one of 1) to 8), wherein at least one ofthe electrodes is a ring-shaped electrode having an opening on its innerside.

10)

A photosensitive device comprising the photodiode according to any oneof 1) to 9).

Advantageous Effects of Invention

According to the technique of the present disclosure, it is possible toprovide a photodiode using a pn junction of Mg₂Si which is a compoundsemiconductor, which has improved overall performance includingphotosensitivity, because the semiconductor photodiode can improve theadhesion between Mg₂Si and the electrode material, as well assignificantly increase photo carriers that can reach the electrode fromMg₂Si, in particular electron holes that can reach the electrode fromp-type Mg₂Si.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a basic structure of an Mg₂Si photodiode.

FIG. 2 shows an energy level diagram for a conventional Ti electrode.

FIG. 3 shows an energy level diagram in the technique of the presentdisclosure.

FIG. 4 shows spectral sensitivity spectra of Example 1 and ComparativeExample 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A photodiode according to the technique of the present disclosurebasically includes: a pn junction of Mg₂Si crystal; an electrodecontaining a material that is in contact with p-type Mg₂Si; and anelectrode containing a material that is in contact with n-type Mg₂Si.Preferably, the Mg₂Si is composed of a crystalline material and issingle crystal. Non-doped Mg₂Si generally exhibits n-type conductivity,and the Mg₂Si having the pn junction of Mg₂Si formed such as byintroducing a p-type impurity into a part of the Mg₂Si is regarded as amain structure for the diode, and the main structure is provided withelectrodes for extracting photocurrent to make the photodiode of thepresent disclosure.

The photodiode according to the present disclosure is characterized inthat a material of the electrode that is in direct contact with p-typeMg₂Si has a work function of 4.81 eV or more, and reacts with Si to forma silicide or form an alloy with magnesium. As described above, theelectron affinity for Mg₂Si is 4.51 eV, and its work function variesdepending on the carrier concentration and is between 4.51 and 4.81 eVfor n-type Mg₂Si and between 4.81 and 5.11 eV for p-type Mg₂Si.Therefore, if the material of the electrode that is in contact withp-type Mg₂Si has a work function of 4.81 eV or more, an energy barriercan be eliminated at an interface between the p-type Mg₂Si and theelectrode for electron holes to be transported to the electrode, amongelectron hole pairs of photo carriers formed in a depletion layer and adiffusion region of the pn junction, or an impact of the energy barriercan be reduced to such an extent that causes no practical problems, sothat a collection efficiency of carriers can be significantly improved.

The material of the electrode that is in direct contact with p-typeMg₂Si has a work function of 4.81 eV or more as described above, andreacts with Si to form a silicide or forms an alloy with magnesium.Thus, when the material of the electrode that is in direct contact withMg₂Si is the material that reacts with Si to form a silicide or an alloywith magnesium, a part of Si in Mg₂Si can be allowed to react with theelectrode material at the interface where Mg₂Si is in contact with apart of the electrode material to form a strong bond, resulting inhigher adhesion of p-type Mg₂Si to the electrode enough to withstand theuse as a practical device.

In the photodiode according to the present disclosure, examples of theelectrode material that is in direct contact with p-type Mg₂Si, whichhas a work function of 4.81 eV or more and reacts with Si to form asilicide or form an alloy with magnesium, includes, at least one metalselected from the group consisting of nickel (Ni), cobalt (Co), platinum(Pt), palladium (Pd), iridium (Ir), rhenium (Re), rhodium (Rh),beryllium (Be), selenium (Se), and tellurium (Te), or at least one alloythereof. The use of such a metal material as the electrode material thatis in direct contact with p-type Mg₂Si can lead to improved adhesion atthe interface between the metals in the case of an electrode structurewhere the metal material is further provided with other metal material.

As described above, the energy barrier formed at the interface betweenp-type Mg₂Si and the electrode that is in direct contact with p-typeMg₂Si is problematic in the prior arts, but the same problem is causedat the interface between n-type Mg₂Si and the electrode that is indirect contact with the n-type Mg₂Si. Since majority carriers will beelectrons at the interface between n-type Mg₂Si and the electrode, thework function of the electrode material that is in direct contact withn-type Mg₂Si should be 4.81 eV or less which is the work function ofn-type Mg₂Si, in order to prevent formation of the energy barrier forelectrons transported to the electrode at the interface. Moreover, theelectrode material is formed as a material that reacts with silicon toform a silicide or form an alloy with magnesium, whereby good adhesionto Mg₂Si can be obtained.

As described above, examples of the material that has a work function of4.81 eV or less and reacts with Si to form a silicide or form an alloywith magnesium include at least one metal selected from the groupconsisting of aluminum (Al), silver (Ag), titanium (Ti), gallium,indium, arsenic, antimony, bismuth, copper, zinc, cadmium, vanadium (V),chromium (Cr), manganese (Mn), iron (Fe), yttrium (Y), zirconium (Zr),niobium (Nb), molybdenum (Mo), hafnium (Hf), tantalum (Ta) and tungsten(W), or at least one alloy thereof. In the prior art Non-Patent Document3, Ag having a work function of 4.26 eV is used as an electrode materialthat is in direct contact with n-type Mg₂Si.

In the photodiode according to the present disclosure, the electrodethat is in contact with p-type Mg₂Si may be an electrode having astructure including: a material that is in direct contact with p-typeMg₂Si; and other material that is in contact with the material. Such astructure can allow characteristics of the entire electrode to bechanged to desired characteristics to further improve thecharacteristics, as compared with a case where the electrode material ismade of only one material that is in direct contact with p-type Mg₂Si.For example, electrical properties, durability and weather resistance ofthe entire electrode can be improved by using an electrode formed bycombining materials having higher conductivity than the material that isin direct contact with p-type Mg₂Si or combining materials having highchemical stability.

Examples of such materials include at least one metal selected from thegroup consisting of Au, Pd, and Pt or at least one alloy thereof. Astructure formed such that only Au or Pd or its alloy portion of theelectrodes is exposed by means of an appropriate passivation treatmentcan allow prevention of the electrode material that is in direct contactwith p-type Mg₂Si from being degraded to achieve a photodiode havinghigh durability and weather resistance, even if the electrode materialis a material easily degraded by oxidation or the like.

Further, in the photodiode according to the present disclosure, aspecific arrangement form and the like of the material that is in directcontact with p-type Mg₂Si is not particularly limited, including, as aspecific aspect, a thin film having a thickness of from 1 to 1000 nm. Ifthe thickness is less than 1 nm, any sufficient adhesion to p-type Mg₂Simay not be obtained. If the thickness is more than 1000 nm, theelectrode may be easily separated, or the electrical resistance of theelectrode may be increased, which may cause a decrease in photocurrent.The thickness can be 5 nm or more, or 8 nm or more, and 500 nm or less,or 100 nm or less.

As described above, the pn junction of Mg₂Si crystal, which is the mainstructure of the photodiode according to the present disclosure, can beformed by doping a part of non-doped Mg₂Si crystal that will generallybe of n-type, with an impurity that will be of p-type, as describedabove. Such a dopant impurity includes Ag. Ag is an element whichdiffuses into the interior of the Mg₂Si crystal by a heat treatment toform relatively easily a Mg₂Si structure locally doped with Ag.

Furthermore, in the photodiode according to the present disclosure, aspecific structure, shape and the like of the electrode are notparticularly limited, and examples of the structure and the likeinclude, in a specific embodiment, a ring-shaped electrode having anopening on its inner side, as an electrode on at least one side. Asdescribed above, when the ring-shaped electrode having an opening in itsinner side is formed on a surface on at least one side of the structurewhere the pn junction is formed at a predetermined position in a depthdirection from the surface of Mg₂Si, a light passing through the openingcan excite photo carriers in a wide range of pn junctions, so that aphotodiode having high photosensitivity can be realized.

In addition, the structure of FIG. 1 shows that the electrode on theside that is in contact with p-type Mg₂Si is in the ring shape, but thepresent disclosure is not limited to this structure, and the electrodeon a side that is in contact with n-type Mg₂Si may be in the ring shapeor the electrodes on both sides may be in the ring shape. Further, asused herein, the “ring shape” is not limited to a circular shape, andmeans an annular shape including oval and polygonal shapes.

Various photosensitive devices such as a photodetectors and imagingdevices can be formed by using the Mg₂Si photodiode having the structureas described above as a basic element structure. In particular, theMg₂Si photodiode according to the present disclosure has goodsensitivity characteristics for infrared light in a wavelength range offrom 900 to 1900 nm, among the short wavelength infrared ranges, and canbe suitably used for a photosensitive device assuming the use in such awavelength range.

A method for producing the photodiode according to the presentdisclosure is not particularly limited, and the photodiode according tothe present invention may be produced by a method including any means aslong as the above structure of the photodiode can be realized.Hereinafter, an example of the production method and technical meansrelating to the production which can be used to embody the structure ofthe photodiode according to the present disclosure are shown, althoughnot limited thereto.

First, to form the pn junction of Mg₂Si crystal, which will be a mainstructure of the photodiode according to the present disclosure, acrystalline material of Mg₂Si is prepared. The single crystal materialof Mg₂Si is preferable as the crystalline material of Mg₂Si as describedabove, and the single crystal material of Mg₂Si can be obtained by aknown method as disclosed in, for example, Non-Patent Document 1. Inorder to form the photodiode according to the present disclosure, it ispreferable that the Mg₂Si crystalline material is formed into aplate-shaped substrate having a thickness of from about 0.1 to 5 mm inadvance, and is then used after polishing its surface, in terms of theprocess.

A part of the Mg₂Si crystalline material prepared according to the aboveprocess is doped with a p-type impurity to form a pn junction of Mg₂Si.Since non-doped Mg₂Si exhibits n-type conductivity, the doping of thepart of non-doped Mg₂Si with the p-type impurity to change a partialregion of the crystalline material of Mg₂Si to p-type Mg₂Si will resultin formation of pn junction at an interface between the doped region andthe non-doped region.

The means for doping a part of the crystalline material of Mg₂Si withthe p-type impurity and the p-type impurity species (dopant) are notparticularly limited, and desired means and dopants may be used. Herein,the use of Ag as a dopant and a doping method by thermal diffusion aregiven as an example. Ag is disposed as a diffusion source on the surfaceof the crystalline material of Mg₂Si, and heated in an inert atmosphereto diffuse thermally Ag from the surface of the crystalline material ofMg₂Si to the interior. Ag as a diffusion source can be arranged andformed on the surface of the crystalline material of Mg₂Si by vacuumdeposition, sputtering or the like, in an amount required for thermaldiffusion to the interior. The conditions of the heat treatment asdescribed above can be adjusted and set in view of a diffusion rate anda depth of a diffusion region to be formed, i.e., a position where thepn junction is formed. For example, the heat treatment temperature maybe set to 400 to 550° C., and the heat treatment time may be set withina range of from 30 seconds to 30 minutes.

An Electrode required for extracting and detecting photocurrent is thenformed in each of the p-type and n-type regions of the Mg₂Si crystal inwhich the pn junction is formed. A specific means for forming theelectrode on the surface of the Mg₂Si crystal in each of the p-type andn-type regions is not particularly limited, and the electrode may beformed by using known methods such as vacuum evaporation, sputtering andplating depending on the electrode material and the like. In this case,a masking or photolithographic technique may also be used to form aring-shaped electrode or a desired electrode pattern, as disclosed inNon-Patent Documents 2 and 3.

In addition to the foregoing, further operation may be additionallycarried out as needed which forms a multilayer electrode by carrying outelectrode formation with other material, or forms a protective layer, orperforming etching or polishing to remove a part of an unnecessarystructure. The means and conditions specifically mentioned above aremerely by way of example, and other means and conditions may be appliedas long as the essential structure of the Mg₂Si photodiode according tothe present disclosure can be obtained.

EXAMPLES

The technical contents of the present disclosure will be specificallydescribed below based on Example and Comparative Example. The followingExample and Comparative Example are merely specific examples for betterunderstanding of the technical contents of the present disclosure, andthe scope of the present invention is not limited by these specificexamples.

Example 1

A single crystal material of n-type Mg₂Si grown by a vertical Bridgmanmethod according to the method disclosed in Non-Patent Document 1 wasprepared as a crystalline material of Mg₂Si, and the single crystalmaterial was cut out at a (110) plane to mirror-polish both sides. Afterpolishing, the material was washed to obtain a Mg₂Si single crystalsubstrate having a thickness of 1 mm. The carrier concentration of thesubstrate is 6×10¹⁵ cm⁻³. An Ag layer having a diameter of 800 μmserving as a diffusion source was formed on a part of one surface of thesubstrate by a vacuum deposition method, and then thermally diffusedfrom one surface of the Mg₂Si substrate in the depth direction bycarrying out a heat treatment in an argon (Ar) atmosphere at 450° C. for10 minutes to form a p-type Mg₂Si layer in a partial region of then-type Mg₂Si crystal.

Then, on the surface of the formed p-type Mg₂Si layer, a circularring-shaped Ni layer having an inner diameter of 500 μm, a width of 75μm, and a thickness of 10 nm was formed by sputtering. An Au layerhaving a thickness of 300 nm and the same size as that of the Ni layerwas then formed directly on the formed Ni layer by vacuum evaporation.In this example, in the pn junction of Mg₂Si crystal, the material thatis in direct contact with p-type Mg₂Si is Ni having a work function of5.15 eV, and the structure including the Ni layer and the Au layerformed directly thereon is an electrode on the p-type Mg₂Si side.Further, an Al layer having a work function of 4.28 eV was formed so asto have a thickness of 300 nm by vacuum deposition over the entiresurface of the n-type Mg₂Si on the opposite side of the substrate, whichwas regarded as an electrode on the n-type Mg₂Si side. It should benoted that the carrier concentration in the p-type region is 1×10¹⁹ cm⁻³from the above diffusion conditions, and the work function of p-typeMg₂Si at this time is estimated to be about 5.09 eV, and the workfunction of n-type Mg₂Si is estimated to be about 4.62 eV, from thecarrier concentrations in the p-type and n-type regions.

The spectral sensitivity characteristics of the Mg₂Si photodiode thusproduced were evaluated by measuring spectral sensitivity spectrum. Themeasurement was carried out by diffracting a light from a halogen lampby a spectrometer and allowing the light to be incident from the openingside of the ring-shaped electrode to the photodiode formed as describedabove, and amplifying the resulting photocurrent by a circuit using anoperational amplifier, and detecting the photocurrent using a lock-inamplifier.

FIG. 4 (the solid line) shows a spectral sensitivity spectrum at awavelength of 1300 to 2200 nm. It was confirmed from the spectrum thatthe spectral sensitivity had a peak at a wavelength of 1350 nm, and itsmaximum value was about 0.14 NW. The adhesion of the electrodes wasevaluated by a tape test method according to JIS H 8504, indicating thatthe electrodes adhering to the tape were 5% or less of the electrodearea for both pn sides, and no problem of the adhesion of the electrodeswas observed.

Comparative Example 1

A photodiode was produced by the same method as that of Example 1, withthe exception that the material of the electrode layer that was indirect contact with the surface of the p-type Mg₂Si layer was Ti. Thatis, in this Example, the material that is in direct contact with p-typeMg₂Si is a layer of Ti having a work function of 4.33 eV, and thestructure including the Ti layer and an Au layer formed directly on theTi layer is an electrode on the p-type Mg₂Si side in the pn junction ofMg₂Si crystal. An electrode that is in contact with the entire surfaceof the opposing n-type Mg₂Si is Al as in Example 1. In this example, thespectral sensitivity characteristics were also evaluated by the samemeans and conditions as those of Example 1. In addition, as with Example1, the work function of p-type Mg₂Si at this time is estimated to beabout 5.09 eV, and the work function of n-type Mg₂Si is estimated to beabout 4.62 eV from the carrier concentrations in the p-type and n-typeregions.

FIG. 4 (the dotted line) also shows a spectral sensitivity spectrum atwavelength of 1300 to 2200 nm. The shape of the spectrum isapproximately similar to that of Example 1. However, it was confirmedthat the maximum value of the spectral sensitivity at the peak positionwas less than about 0.08 NW, which was less than about 57% of themaximum value in Example 1. In this example, the evaluation result ofthe adhesion of the electrode by the tape test method according to JIS H8504 also indicated that the electrodes on both pn sides were 5% or lessof the electrode area, and no problem of the adhesion of the electrodewas observed.

These results are summarized in Table 1 below.

TABLE 1 Electrode in contact with Electrode in contact with p-type Mg₂Sin-type Mg₂Si Spectral Sensitivity Material Work Function (eV) AdhesionMaterial Work Function (eV) Adhesion Maximum Value Example 1 Ni 5.15 ◯Al 4.28 ◯ 0.14 Comparative Example 1 Ti 4.33 ◯ Al 4.28 ◯ 0.08

As can be seen from the above results, no significant problem isrecognized in both Examples in relation to the adhesion of theelectrodes to Mg₂Si. This would be because both materials that are indirect contact with Mg₂Si react with Si to form a silicide. However, forthe values of the spectral sensitivities, a significant difference wasobserved between Example and Comparative Example. It is believed that,in Example 1 where the material that is in direct contact with p-typeMg₂Si is Ni having a work function of 5.15 eV, no energy barrier fortransportation of electron holes is formed at the interface betweenp-type Mg₂Si and Ni electrode layer, so that the electron holes asmajority carriers can effectively reach the electrodes.

On the other hand, it is believed that, in Comparative Example 1 wherethe material that is in direct contact with p-type Mg₂Si is Ti having awork function of 4.33 eV, even if photo carriers are generated by lightincidence, a part of the electron holes is prevented from transported tothe electrode due to the energy barrier formed at the interface betweenthe p-type Mg₂Si and the Ti electrode layer, so that the photo carriersare not effectively detected as photoelectric current. In view of these,it has been found that it is highly effective to select and adjustappropriately the work function of the material that is in directcontact with p-type Mg₂Si based on the technical idea according to thepresent disclosure, in order to improve the performance of the Mg₂Siphotodiode significantly.

INDUSTRIAL APPLICABILITY

According to the technique of the present disclosure, in the photodiodeusing the pn junction of Mg₂Si, the light sensitivity can besignificantly improved as compared with the prior arts. Therefore, it ispossible to improve dramatically the performance of various devices forsensing and imaging in the infrared regions at short wavelengths(approximately from 0.9 to 2.5 μm) assumed by photodiodes using Mg₂Si.Accordingly, a significant contribution can also be expected totechniques of various image analysis and image diagnosis in thosewavelength ranges, and further automatic monitoring and automaticcontrol techniques using them, as well as industrial fields using thesetechniques.

DESCRIPTION OF REFERENCE NUMERALS

-   104 other material electrode that is in contact with material that    is in direct contact with p-type Mg₂Si-   301 depletion layer and diffusion length-   302 electron-   303 electron holes

1. A photodiode comprising: a pn junction of a magnesium silicidecrystal; an electrode comprising a material that is in contact withp-type magnesium silicide; and an electrode comprising a material thatis in contact with n-type magnesium silicide, wherein the material thatis in contact with p-type magnesium silicide is a material which has awork function of 4.81 eV or more and reacts with silicon to form asilicide or form an alloy with magnesium, and the material that is incontact with p-type magnesium silicide is at least one metal selectedfrom the group consisting of nickel, cobalt, platinum, palladium,iridium, rhenium, rhodium, beryllium, selenium, and tellurium, or atleast one alloy thereof.
 2. (canceled)
 3. The photodiode according toclaim 1, wherein the material that is in contact with n-type magnesiumsilicide is a material which has a work function of less than 4.81 eVand reacts with silicon to form a silicide or form an alloy withmagnesium.
 4. The photodiode according to claim 1, wherein the materialthat is in contact with n-type magnesium silicide is at least one metalselected from the group consisting of aluminum, gallium, indium,arsenic, antimony, bismuth, silver, copper, zinc, cadmium, titanium,vanadium, chromium, manganese, iron, yttrium, zirconium, niobium,molybdenum, hafnium, tantalum and tungsten, or at least one alloythereof.
 5. The photodiode according to claim 1, wherein the electrodecomprising the material that is in contact with p-type magnesiumsilicide comprises: the material that is in contact with p-typemagnesium silicide; and other material that is in contact with theformer material.
 6. The photodiode according to claim 5, wherein theother material is at least one metal selected from the group consistingof gold, palladium and platinum or at least one alloy thereof, exceptfor the metal selected as the material that is in contact with p-typemagnesium silicide.
 7. The photodiode according to claim 1, wherein thematerial that is in contact with p-type magnesium silicide is in a formof a thin film having a thickness of from 1 to 1000 nm.
 8. Thephotodiode according to claim 1, wherein the p-type magnesium silicideis magnesium silicide doped with silver.
 9. The photodiode according toclaim 1, wherein at least one of the electrodes is a ring-shapedelectrode having an opening on its inner side.
 10. A photosensitivedevice comprising the photodiode according to claim 1.